1. Field of the Invention
The present invention relates to a bioremediation process for degrading pollutants by the utilization of the substance conversion mechanism of microorganisms to make them harmless and to thus decontaminate a soil water system polluted with the various environmental pollutants.
2. Related Background Art
As a result of the diversification of industries and the dumping of wastes without consideration for a natural ecological system, pollutants which are hard to degrade are now accumulated in soils in districts in the world. Accordingly, the pollutants leak into underground water systems, so that the polluted districts are growing larger. In these pollutants which are hard to degrade, substances such as trichloroethylene and PCB which are extremely harmful to human bodies are present, which is a large social problem.
As measures for the decontamination of such a polluted soil water system, techniques which have now been practiced are a method of pumping up groundwater and then exposing it to the atmosphere, a method of subjecting the polluted soil water system to a pollutant adsorption removal treatment with active carbon, a method of extracting volatile pollutants from the soil under reduced pressure, a process of air-drying the polluted soil, or the like.
However, these methods have a problem that the soil must be dug up over an extensive area to remove the pollutants, and a problem that the diffusion of the harmful gas into the atmosphere cannot be completely prevented, and the like. These problems have an adverse influence on the environment.
As understood from the foregoing, the environmental pollutants are composed of various kinds of substances and their amounts are very large, and the polluted ranges also often extend inconveniently. In consequence, in the cause of environmental decontamination, it is not always practical to depend upon physical and chemical means alone in view of the energy and cost required for the decontamination treatment.
Accordingly, much attention has been paid to various substance conversion mechanisms which microorganisms exhibit, and bioremediation is expected in which the environmental pollutants can be degraded by utilizing microorganisms to make them harmless. The substance degrading ability of the microorganisms is based on reaction with the aid of enzymes which they have, and hence, the bioremediation has suitable characteristics from the viewpoint of environmental safeguards. For example, reaction conditions are milder, the consumption of energy is smaller, the production of by-products is smaller, and suitability for an in-situ treatment and anon-site treatment for directly treating a polluted region is better as compared with conventional chemical reaction.
The bioremediation utilizing the microorganisms can fundamentally be classified into two cases. In the first case, the microorganisms originally present in the polluted region are utilized. In this case, the degrading ability of the microorganisms which have naturally grown by utilizing the pollutants as an energy source is extended, and the extended degrading activity is enhanced. In the second case, the microorganisms which are excellent in the pollutant-degrading ability and which are not derived from the polluted region are artificially introduced into the polluted region. As the microorganisms which can be used in this case, there are microorganisms separated from a natural world, or microorganisms in which the pollutant-degrading ability is artificially enhanced by a technique such as gene manipulation or mutagenesis and which can easily be discharged into the environment without any problem.
However, the pollutant degrading activity is not always obtained in the region to be decontaminated by the microorganisms for decontaminating the polluted region which are present in the soil or are newly administered to the soil. Most of the microorganisms are generally passive to the given circumstances and they sensitively react thereto, and for this reason, it is necessary that the circumstantial conditions given to the microorganisms be properly controlled to heighten their decontamination ability. As techniques for this control, there are a method of spraying nutrients for the microorganisms on the soil in order to activate the pollutant-degrading ability of the microorganisms present in the soil or newly administered to the soil, a method of stirring and aerating the soil to make it aerobic, and the like.
However, the soil forms a natural ecological system, and in the soil, there is a food chain constituted of competition between the microorganisms and the hierarchical structure of the predator and the prey microorganisms. Most of the microorganisms which can be expected as the pollutant-degrading microorganisms are mainly bacteria, but in the soil, the bacteria is in a lowest nutrient stratum at which they grow utilizing organic substances derived from plants or animals as a nutrient source. At a stratum above these bacteria, there are protozoa such as rhizopods (amoebae and the like) and ciliates, and they eat the bacteria. Therefore, if the pollutant-degrading bacteria are eaten by these creatures in the soil and the number of the bacteria decreases, the degradation effect of the pollutants noticeably deteriorates. Therefore, it is desired to develop a technique by which the degrading bacteria are protected from being eaten so that the degrading bacteria may be allowed to remain in the soil. However, the bioremediation utilizing the microorganisms has been merely applied for a short period of time since its development, and therefore as the survival technique of the degrading bacteria in the soil, a process has been tried in which the degrading bacteria are supported in a porous carrier or the like to protect the degrading bacteria from being eaten by the protozoa. However, in order to disperse, in the soil, a carrier having a larger particle diameter than the soil particles, an operation such as digging and stirring of the soil which has a large environmental load is not avoidable, and in this case, the feature of the in-situ treatment of the bioremediation cannot be utilized.